One problem faced by designers of grit wheel technology plotters is the prevention of the dynamics of the moving medium (paper, mylar, vellum, etc.) from hindering the writing quality of the plot. This problem has two manifestations, which the invention described herein solves.
The first problem is the formation of waves in the medium, which travel up the platen and interfere with the pen. These waves are large enough to lift the pen from the medium or even strike the pen while it is in the penup position. These phenomona are known, respectively, as the skip and the drag.
The second problem is the scalloping of lines drawn on an angle (angled vectors) during high acceleration. Since the medium is being driven only by its edges parallel to the direction of motion, the center of the medium tends to lag until constant velocity is reached. Although this is only slightly noticeable on vectors longer than 15 mm, it is glaringly obvious on shorter vectors, especially those used to generate characters.
One solution used in response to the medium wave problem is a firmware filter. This filter looks at incoming vectors, and based on an algorithm which is a function of velocity, acceleration, move length, and move direction, slows the plotter down to twice the acceleration of gravity (2 g) from a maximum of 4 g. This solution increases plot time by an average of 10% for typical demonstration plots. This average, however, does not indicate an even distribution. Rather, some plots are clustered around 5% and others around 20%. In the worst case, plot time may be increased by 40%. High throughput in both sheet and roll mode is seriously compromised if much of the plot time is spent at 2 g.
The scalloping problem has also received a firmware fix. Here, the solution consists of pulsing the acceleration on angled vectors so that the pen is actually ahead of its intended position. Consequently, the pen does not compensate enough in the middle of the sheet and overcompensates near the edges under the pinch rollers. This transforms line quality from poor in the center and perfect at the edges to consistently poor everywhere, but marginally passable.
Thus, there is a need to provide a means for simultaneously isolating the moving waves and preventing scalloping of angled vectors during high acceleration.